How to define populations
This is one of the most important concepts to agree on before searching for data. The definition of a population does differ depending on the species (e.g. the spatial extent for a tree population and a salamander may not be the same) but here follows some general guidance.
Briefly, ‘population’ refers to a genetically distinct group of individuals. Within populations the individuals are capable of interbreeding. Genetic distinction of populations is usually due to no or very low gene flow (movement of genes e.g. by movement of offspring) and/or significant adaptive differences, such as due to occurring in different environments (high/ low altitude, different soil types, differences in precipitation or temperature). Not every occurrence is a population! The word ‘subpopulation’ may describe clusters of organisms across a landscape (including family units) which are near enough to exchange gene flow. Usually several ‘subpopulations’ are considered together as a population (which can also be called a ‘metapopulation’). When the potential for gene flow is large, e.g., viable tree pollen that can travel tens of kilometers, ‘populations’ can range across large distances, sometimes hundreds of km.
Some published reports will clearly define what a population is based on the knowledge of biodiversity and taxonomic experts. Some judgment may be required in interpreting population designation in a report, but in general the designation of a population by experts should be considered strongly. In other cases, the reports or database may not clearly designate population boundaries and will require interpretation. Visual examination of maps may result (correctly) in ‘merging’ occurrences that are likely to experience extensive gene flow- ‘likely to experience extensive gene flow’ may be defined on spatial proximity e.g. close geographic distance and lack of clear barrier (mountain, fence, etc.).
The distance between subpopulations (or discrete habitat patches where the species occurs) can be used as a proxy to evaluate to what extent subpopulations are likely functionally connected. When the edge of a subpopulation is within a reasonable dispersal distance (for that particular species, a distance within which some 75-90% of realized dispersal distances occur- expert discretion is fine here) of another subpopulation edge, and there are no known physical barriers impeding dispersal, the subpopulations can be considered as part of the same metapopulation. Designation of populations should consider human induced gene flow e.g. genetic rescue, translocations etc. Experts in the species type (e.g. amphibians) and/or text from the data source can help identify populations.
Some examples may help. A population may consist of a cluster of individuals in a discrete location like an island, lake, river catchment or forest preserve, separate from other discrete locations by some tens of kilometers. A population may constitute a metapopulation consisting of subpopulations (ponds, prairies, etc.) that are separate but not very far (hundreds of meters to several kilometers), thus capable of exchanging at least 1 migrant (one reproductive adult moving between patches) on average per generation with each other. Well connected subpopulations are sufficiently close for the metapopulation to be called a population, and the population size should be considered the sum of the subpopulations, which may cover tens or hundreds of kilometers.
Genetic data can help define populations, but should be used with some caution. Whether genetic data can detect genetic differences can be a consequence of the type of genetic marker- new, genomic level markers can detect very fine genetic divisions, including between subpopulations. Not every genetic distinction according to DNA markers is a population. The threshold should still be considered roughly less than 1 migrant per generation (on average). Older genetic markers such as chloroplast and mitochondrial DNA sequence are often useful in identifying highly genetically distinct populations.
Populations of common species with large continuous spatial distributions that are much larger than the dispersal capacity of a single individual (e.g. less than 1 migrant per generation across the extent) are also considered as metapopulations. This applies to common, abundant species. Many populations of common species will be either nearly continuous or occur over very large areas. When very large continuous populations exist, e.g. hundreds of kilometers, then somewhat arbitrary population boundaries should be instituted, based on ecological changes such asecoregions.
Metapopulations should represent stable spatial and temporal units. Many species have ephemeral subpopulations in dynamic source-sink metapopulations- a sink is a spatial location receiving high immigration from adjacent areas and which would not persist on its own without immigration. Sinks are not distinct populations. The conglomerate of connected subpopulations that should be evaluated as a population.
For freshwater fish, the riverscape structure can help define populations or units that can be assimilated to populations/metapopulations with a little GIS work. Individuals inhabiting lakes can be considered as populations, especially for lakes that are disconnected from the hydrographical network. Riverscape (meta)populations can also be defined through their level of connection/disconnection e.g. belonging to different river basins, hydrographical systems, river stretches separated by huge dams, etc.
For trees, the pollination mode and commonness is important. Trees which are wind-pollinated can have continuous populations extend over tens of kilometers, and farther. Trees which are insect pollinated generally but not always, have less gene flow. For common trees, a distinct population may not be easily apparent (e.g. trees that extend across much of a continent in a continuous fashion). In such cases, a “population” may be considered at approximately the level of a state, country, or ecoregion (hundreds of kilometers across).